1) Field of the Invention
This invention relates to a method of actually measuring a pressure-current characteristic as an operation characteristic of a proportional electromagnetic control valve in a hydraulic circuit or a velocity-current characteristic of a hydraulic cylinder; a method of controlling, based on thus measured result, an operation state of the hydraulic cylinder; and a method of modifying a prestored operation characteristic. In particular, the present invention relates to a method of measuring an operation characteristic of a proportional electromagnetic control valve, a method of controlling an operation of a hydraulic cylinder, and a method of modifying an operation characteristic of the proportional valve which are suitably used when an injection molding machine is inspected at its manufacturing site upon shipping or when the injection molding machine is controlled while being actually operated.
2) Description of the Related Art
In an injection molding machine, while a hydraulic pressure from a hydraulic pump or accumulator is supplied to a hydraulic cylinder (injection cylinder, closing cylinder, nozzle tcuch cylinder, or the like) via a proportional electromagnetic control valve (proportional electromagnetic flow control valve or proportional electromagnetic pressure control valve) so as to control velocity or pressure, such a proportional electromagnetic control valve yields a large fluctuation in its characteristic in general.
Consequently, prior to shipment of an injection molding machine, its hydraulic circuit is operated in the actual machine, several points of pressure levels or flow levels of its proportional electromagnetic control valve with respect to its load current are measured, thus measured levels are compared with a reference characteristic attached to the proportional electromagnetic control valve, and the gain of the load current output circuit for the proportional electromagnetic control valve is manually adjusted before shipping. Inmost cases, however, only the maximum and minimum levels are measured for output current adjustment and, also under the influence of differences among machines and circuit configurations, thus modified characteristic may not have a high accuracy.
Japanese Patent Publication No. SHO 64-9177 discloses a technique in which, in order to enhance accuracy of an actually employed characteristic, the characteristic of a proportional electromagnetic control valve built in an injection molding machine is actually measured before shipping its hardware, a correction characteristic table providing a relationship between an ordered level and an output level is prepared according to the measured result, and the proportional electromagnetic valve is controlled with reference to the contents of this correction characteristic table upon actually operating the hardware.
In the characteristic auto-modifying technique for a proportional electromagnetic control valve disclosed in the above-mentioned publication, the proportional electromagnetic control valve is operated with its input levels successively changing from the minimum input level to maximum input level so that their corresponding numeral output levels are initially measured and, according to a result of a comprehended relationship between these numerous actually measured levels and their corresponding ordered levels, the correction characteristic table is prepared such that the output level is proportional to the input level.
In such a technique for measuring the characteristic of a proportional electromagnetic control valve, while numerous points from the minimum input level to the maximum input level are actually measured and plotted when preparing the correction characteristic table, there may be cases where, due to the condition under which a hydraulic cylinder is used and the like, the maximum pressure or maximum velocity cannot be attained in the state where the proportional electromagnetic control valve is built in the machine. In such a case, the characteristic of the proportional electromagnetic valve cannot be modified. Also, the number of measuring points may be so large that an enormous amount of labor and time is necessitated.
Theretfore, Japanese Patent Application Laid-Open (Kokai) No. HEI 5-57777 discloses a technique in which reference characteristic curve data of a proportional electromagnetic control valve of the same type as that of the proportional electromagnetic control valve built in the machine are prestored in a storage circuit in a microcomputer in a control unit of the injection molding machine, the microcomputer causes the proportional electromagnetic control valve to operate (in a test run) by given ordered levels at predetermined two points, output (pressure or velocity) levels corresponding thereto are respectively measured, and then, according to the two points of ordered levels and output levels obtained by the test run and the above-mentioned reference characteristic curve data, output levels corresponding to individual points on the reference characteristic curve are successively computed to recompose the characteristic curve.
In the technique in which the actually measured level and the reference characteristic table attached to the proportional electromagnetic control valve are compared with each other, and the maximum and minimum levels are regulated by gain adjustment of a load current output circuit amplifier of the proportional electromagnetic control valve so as to modify characteristic levels of the proportional electromagnetic control valve, as mentioned above, however, data of proportional electromagnetic control valve differ among different machines and, also under the influence of differences in circuit configurations, a linearity may not be obtained between the ordered and executed levels, thus deteriorating the accuracy of the modified characteristic.
In the technique in which a characteristic is actually measured, a correction characteristic table keeping a relationship between ordered and output levels is prepared, and a correction characteristic is added so as to control the proportional electromagnetic control valve as disclosed in Japanese Patent Publication No. 64-9177, numerous points from the minimum input level to maximum input level must be actually measured to prepare the correction characteristic table as mentioned above. Here, since the number of measured points is large, an enormous amount of labor and time is necessary.
In the method disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 5-57777 in which characteristic data of the proportional electromagnetic control valve are obtained on the analogy of two points of data, correct data may not be obtained from the two points alone since the original data are a curve constituted by a polynomial function.
In the foregoing method in which a data table is retrieved, an accurate resolution as fine as 0.1% cannot be obtained.
In the conventional pressure measurement in an actual hydraulic circuit, in the case where pressure is relatively low, the pressure is measured after a rising overshoot upon switching hydraulic pressure is suppressed. When the number of measuring points is large, however, it takes much time therefor. When measuring velocity, since time by which a predetermined position travels a predetermined distance is measured to determine the velocity. Accordingly, it takes much time for measurement at the minimum velocity, and an enormous amount of time is required for measuring numerous points.